This invention relates to an appatauus and a method for producing a coated optical fiber by obtaining an optical fiber by drawing, heating and spinning optical fiber base material and thereafter coating the optical fiber with a resin and more particularly to an apparatus and a method for producing a coated optical fiber adapted to cool the optical fiber before it is coated with the resin while cooling the optical fiber is so controlled that it is stably cooled down even though the line speed or drawing speed of the optical fiber varies.
The line speed or drawing speed of the optical fiber is required to increase in order to improve a productivity of the coated optical fiber and lower the production cost when it is produced. As the line speed of the optical fiber increases, the optical fiber is introduced into a coating device before it is cooled down to a predetermined temperature in the cooling assembly. Since a thickness of a resin coat around the optical fiber depends on the temperature of the optical fiber introduced into the coating device, the higher temperature of the optical fiber causes the optical fiber to have the resin coat of less than predetermined thickness. Thus, as the line speed of the optical fiber increases, a cooling capacity of the cooling assembly should be improved.
In general, an He gas of high cooling efficiency has been used as cooling medium to be supplied to the cooling assembly. The He gas is retrieved out of the cooling assembly and then exhausted to the atmosphere. Thus, the production cost will be disadvantageously higher because the consumption volume of the He gas increases as the line speed of the optical fiber increases.
In order to avoid such a drawback, there has been proposed an apparatus for producing a coated optical fiber of cooling gas circulation system wherein an He gas sucked out of a cooling assembly is purified and collected to be a recovery gas and recycled to the cooling assembly (see Japanese Laying-Open No. 240,129/1992 or 4-240,129, for example).
As shown in FIG. 7, this apparatus for producing the coated optical fiber comprises a drawing furnace in which an optical fiber base material 1 is drawn while being heated and spun to obtain an optical fiber 3, a cooling assembly 4 through which an He gas flows to cool the thus obtained optical fiber 3, a resin coating device 5 to form a resin coat around the optical fiber cooled down to the predetermined temperature, a curing means 6 to cure the resin coat around the optical fiber 3 and a winding means 9 to wind the coated optical fiber 7 introduced out of the curing means 6 through a guide roll 8.
The He gas is supplied through a gas inlet 4b provided in the cooling assembly 4 at its lower side and sucked out of a gas outlet 4a provided in the cooling assembly 4 at its upper side. The He gas is mixed with an ambient air possibly sucked into a fiber inlet 4c through which the optical fiber 3 is introduced into and also into a fiber outlet 4d through which the optical fiber 3 is taken out of the cooling assembly 4. The thus obtained gas mixture including the He gas and the possible ambient air is sucked out of the gas outlet 4b by a compressor described later.
A gas purifying and circulating means 10 is connected between the gas outlet 4a and the gas inlet 4b of the cooling assembly 4. The gas purifying and circulating means 10 comprises a gas purifier 12 to receive the gas mixture retrieved out of the gas outlet 4a of the cooling assembly 4 through a compressor 11 and separate the air out of the gas mixture to collect the He gas as a recovery gas of He and a gas mixer 14 to mix the recovery gas of He supplied from the gas purifier 12 with a pure He gas to form a supply gas. The gas mixer 14 is connected to the gas inlet 4b of the cooling assembly 4 so that the supply gas is supplied to the cooling assembly 4.
However, since the coated optical fiber production apparatus having such a gas purifying and circulating means 10 is provided with the compressor 11 for retrieving or sucking the gas mixture out. of the cooling assembly 4, if the line speed of the optical fiber 3 is relatively lower and therefore the flow of He gas supplied to the cooling assembly 4 is lower than a flow of the gas mixture sucked by the compressor 11, a purification efficiency of He gas is lowered for the reasons described hereinjustbelow.
The compressor 11 generally has a constant flow quantity of suction of gas. Therefore, as the drawing speed (line speed) of the optical fiber 3 is relatively lower and therefore the flow quantity of He gas supplied thereto is relatively lower, the cooling assembly 4 tends to have a negative pressure applied thereto because a flow quantity of the cooling gas through the cooling assembly 4 is less than the flow quantity of suction of gas by the compressor 11. This causes the He gas in the cooling assembly 4 to be sucked out of the gas outlet 4a of the cooling assembly 4 by the compressor 11 while possibly accompanied by an ambient air drawn through the fiber inlet 4c and the fiber outlet 4d in the cooling assembly 4. Accordingly,the He gas concentration of the gas mixture sucked out of the gas outlet 4a will be lowered so as to lower the cooling capacity and therefore the quantity of consumption of He gas will increase. Furthermore, the lowered He gas concentration of the gas mixture causes the purification efficiency (separation efficiency) of the gas purifier 12 to be disadvantageously lowered.
Therefore, the flow quantity of the gas mixture sucked by the compressor 11 is required to be adjusted to the line speed of the optical fiber 3, but since the adjustment of the flow quantity of suction cannot be accomplished unless the compressor changed, it will be understood that the flow quantity of suction cannot be practically adjusted.
In addition thereto, as aforementioned, the thickness of the resin coat around the optical fiber 3 is affected by the temperature of the optical fiber 3 passing through the cooling assembly 4 and sucked out of the fiber outlet 4d therein and the temperature of the optical fiber 3 is affected by the He gas concentration and a flow rate of a cooling gas supplied to the cooling assembly 4. Thus, unless the He gas concentration and the flow rate of the recovery gas of He obtained by being separated and purified from the gas mixture by the gas purifier 12 are measured, the flow rate of the recovery gas of He and the flow rate of the pure He gas supplied from a pure He gas supply 15 cannot be effectively controlled. As a result, the coat diameter of the resin coat around the optical fiber largely varies.